Luminous Sensitivity and Quantum Efficiency

The radiant sensitivity of a detector - or its quantum efficiency - is one of the most important parameters for TCSPC application. Unfortunately absolute measurements of the radiant sensitivity or the quantum efficiency are extremely difficult. The problem is not only that a calibrated light source or a calibrated reference detector are required but also that extremely low light intensities have to be used. However, accurate attenuation of light by many orders of magnitude is difficult. 

The Anode Luminous Sensitivity is measured in a similar setup as the Cathode Luminous Sensitivity. However, the PMT is operated in the normal way, i.e. by applying the specified voltage distribution to the dynode chain (Fig. 6.27, right). Of course, a calibrated filter has to be inserted in the light path to attenuate the light of the lamp to a level that does not overload the PMT. As mentioned before, the anode luminous sensitivity is not very useful in characterising a PMT for TCSPC. 

The Cathode Radiant Sensitivity is the current of the photocathode divided by the power of the incident light at a given wavelength. Measuring the Cathode Radiant Sensitivity requires a lamp, a monochromator and a reference detector, e.g. a calibrated photodiode. The setup is difficult to calibrate due to the various error sources. 

A technique for measuring the quantum efficiency of a photon counting detector without a calibrated reference detector is described in [301, 356, 357, 358, 423, 536]. The technique is based on the generation of photon pairs - or entangled photons - by parametric down-conversion. The principle is shown in Fig. 6.28. 

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